Pasi Huovinen

Radial and elliptic flow in Pb + Pb collisions at energies available at the CERN Large Hadron Collider from viscous hydrodynamics

A comprehensive viscous hydrodynamic fit of spectra and elliptic flow for charged hadrons and identified pions and protons from Au+Au collisions of all centralities measured at the Relativistic Heavy Ion Collider is performed and used as the basis for predicting the analogous observables for Pb+Pb collisions at the Large Hadron Collider at sqrt(s)=2.76 and 5.5 A TeV. Comparison with recent measurements of the elliptic flow of charged hadrons by the ALICE experiment shows that the model slightly over-predicts the data if the same (constant) specific shear viscosity eta/s is assumed at both collision energies. In spite of differences in our assumptions for the equation of state, the freeze-out temperature, the chemical composition at freeze-out, and the starting time for the hydrodynamic evolution, our results agree remarkably well with those of Luzum [M. Luzum, Phys. Rev. C 83, 044911 (2011)], indicating robustness of the hydrodynamic model extrapolations. Future measurements of the centrality and transverse momentum dependence of spectra and elliptic flow for identified hadrons predicted here will further test the model and shed light on possible variations of the quark-gluon transport coefficients between RHIC and LHC energies.

Hydrodynamical description of 200-A/GeV/c S + Au collisions: Hadron and electromagnetic spectra

We study relativistic S+Au collisions at 200A GeV/c using a hydrodynamical approach. We test various equations of state (EOS{close_quote}s), which are used to describe the strongly interacting matter at densities attainable in the CERN-SPS heavy ion experiments. For each EOS, suitable initial conditions can be determined to reproduce the experimental hadron spectra; this emphasizes the ambiguity between the initial conditions and the EOS in such an approach. Simultaneously, we calculate the resulting thermal photon and dielectron spectra, and compare with experiments. If one allows the excitation of resonance states with increasing temperature, the electromagnetic signals from scenarios with and without phase transition are very similar and are not resolvable within the current experimental resolution. Only EOS{close_quote}s with a few degrees of freedom up to very high temperatures can be ruled out presently. We deduce an upper bound of about 250 MeV for the initial temperature from the single photon spectra of WA80. With regard to the CERES dilepton data, none of the EOS{close_quote}s considered, in conjunction with the standard leading order dilepton rates, succeed in reproducing the observed excess of dileptons below the {rho} peak. Our work, however, suggests that an improved measurement of the photon and dilepton spectra hasmorexa0» the potential to strongly constrain the EOS. {copyright} {ital 1997} {ital The American Physical Society}«xa0less

Elliptic flow in Pb+Pb collisions at p sNN = 2.76 TeV: hybrid model assessment of the first data

We analyze the elliptic flow parameter v{sub 2} in Pb+Pb collisions at {radical}(s{sub NN}) = 2.76 TeV and in Au+Au collisions at {radical}(s{sub NN})=200 GeV using a hybrid model in which the evolution of the quark gluon plasma is described by ideal hydrodynamics with a state-of-the-art lattice QCD equation of state, and the subsequent hadronic stage by a hadron cascade model. For initial conditions, we employ Monte Carlo versions of the Glauber and the Kharzeev-Levin-Nardi models and compare results with each other. We demonstrate that the differential elliptic flow v{sub 2}(p{sub T}) hardly changes when the collision energy increases, whereas the integrated v{sub 2} increases due to the enhancement of mean transverse momentum. The amount of increase of both v{sub 2} and mean p{sub T} depends significantly on the model of initialization.

Quark Polarization in a Viscous Quark-Gluon Plasma

Nuclear Science Division, MS 70R0319, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA(Dated: August 30, 2011)Quarks produced in the early stage of non-central heavy-ion collisions could develop a global spinpolarization along the opposite direction of the reaction plane due to the spin-orbital coupling viaparton interaction in a medium that has finite longitudinal flow shear along the direction of theimpact parameter. We study how such polarization evolves via multiple scattering in a viscousquark-gluon plasma with an initial laminar flow. The final polarization is found to be sensitive tothe viscosity and the initial shear of local longitudinal flow.

Elliptic flow in U+U collisions at p sNN = 200 GeV and in Pb+Pb collisions at p sNN = 2.76 TeV: Prediction from a hybrid approach

We predict the elliptic flow parameter v_2 in U+U collisions at sqrt{s_{NN}}=200 GeV and in Pb+Pb collisions at sqrt{s_{NN}} = 2.76 TeV using a hybrid model in which the evolution of the quark gluon plasma is described by ideal hydrodynamics with a state-of-the-art lattice QCD equation of state, and the subsequent hadronic stage by a hadron cascade model.

Is there elliptic flow without transverse flow

Azimuthal anisotropy of final particle distributions was originally introduced as a signature of transverse collective flow. We show that finite anisotropy in momentum space can result solely from the shape of the particle emitting source. However, by comparing the differential anisotropy to recent data from STAR collaboration we can exclude such a scenario, but instead show that the data favour strong flow as resulting from a hydrodynamical evolution.

Sensitivity of electromagnetic spectra to equation of state and initial energy density in the Pb + Pb collisions at SPS

Abstract We study Pb + Pb collisions at 158 A GeV/ c using hydrodynamical approach. We test different equations of state (EoSs) and different initial conditions and show that there are more than one initial state for each EoS which reproduce the observed hadronic spectra. We also find that different equations of state favor different freeze-out temperature. Simultaneously we calculate the thermal dilepton and photon spectra for each EoS and initial state. We compare the dilepton mass spectrum to data measured by the CERES collaboration and find that the differences in spectra obtained using different EoSs and initial states are not resolvable within the current experimental resolution. However, at invariant masses over 2 GeV the difference in the yield due to various initial states is close to an order of magnitude. We also study the rapidity distribution of lepton pairs and find that for masses around 800 MeV the shape of the distribution depends strongly on the EoS.

Influence of temperature-dependent shear viscosity on elliptic flow at backward and forward rapidities in ultrarelativistic heavy-ion collisions

We explore the influence of a temperature-dependent shear viscosity over entropy density ratio

Dynamical freeze-out criterion in a hydrodynamical description of Au + Au collisions at

eta/s

\sqrt{s_\mathrm{NN}}=200

on the azimuthal anisotropies v_2 and v_4 of hadrons at various rapidities. We find that in Au+Au collisions at full RHIC energy,